PTC Devices and their preparation

ABSTRACT

The flex life of devices comprising a relatively inflexible PTC conductive polymer element is improved by surrounding the PTC element with a layer of a relatively flexible polymeric composition which is melt-fused thereto. Particularly useful devices are self-limiting strip heaters in which the polymeric component of the PTC element comprises a mixture of two crystalline polymers having substantially different melting points, the higher melting polymer having a melting point of at least 160° C., preferably at least 200° C., for example a mixture of polyvinylidene fluoride and a copolymer of ethylene and tetrafluoroethylene. Such devices can be made by melt-extruding a layer of the relatively flexible polymeric composition around the PTC element, and heating to cause melt-fusion of the polymers at the interface of the layer and the PTC element.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of my application Ser. No. 150,910,filed May 19, 1980, now U.S. Pat. No. 4,334,351.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrical devices containing conductivepolymer PTC elements, in particular self-limiting strip heaters, andtheir preparation.

2. Summary of the Prior Art

Self-limiting strip heaters are well-known. They comprise an element ofa PTC conductive polymer having parallel electrodes embedded therein andan outer jacket of one or more layers of an insulating polymersurrounding the PTC element. Other electrical devices comprising a PTCelement, one or more electrodes and an encapsulating jacket of aninsulating polymer are also known. In the preparation of such devices,the device is often subjected to an annealing step in which it is heatedto a temperature above the melting point of the PTC element in order toreduce the resistivity of the PTC composition. Reference may be made forexample to U.S. Pat. Nos. 3,793,716; 3,823,217 (Kampe); 3,861,029(Smith-Johannsen et al.); 3,914,363 (Bedard et al.) and 4,177,376(Horsma et al.) and to commonly assigned U.S. patent applications Ser.Nos. 84,352 (Horsma et al.); 88,344 (Lutz) now abandoned and the CIPthereof Ser. No. 134,354; 732,792 (Van Konynenburg et al.), nowabandoned; 751,095 (Toy et al.), now abandoned; 798,154 (Horsma), nowabandoned; 965,343 (Van Konynenburg et al.), now U.S. Pat. No.4,237,441; 965,344 (Middleman et al.), now U.S. Pat. No. 4,238,812;965,345 (Middleman et al.), now abandoned; and 75,413 (Van Konynenburg);and the eight applications filed Apr. 21, 1980 by Gotcher et al. Ser.No. 141,984; Middleman et al. Ser. No. 141,987; Fouts et al. Ser. No.141,988; Evans Ser. No. 141,989; Walty Ser. No. 141,990; Fouts et al.Ser. No. 141,991; Middleman et al. Ser. No. 142,053 and Middleman et al.Ser. No. 142,054. The disclosure of each of these patents andapplications is incorporated herein by reference.

SUMMARY OF THE INVENTION

It is often important that such devices, in particular strip heaters,should not be damaged by flexing. In the further development of suchdevices it has been found desirable to make use of PTC conductivepolymer compositions which are relatively inflexible (i.e. have lowelongations and/or are prone to cracking or other forms of damage inflex tests which are used to determine whether the devices will performsatisfactorily under service conditions). I have found that bysurrounding a PTC conductive polymer element by a layer of a secondpolymeric composition having a substantially greater flexibility and byheating the device so that there is melt fusion of polymers at theinterface between the layer and the PTC element, a device havingimproved resistance to damage by flexing is obtained.

In one aspect the invention provides a flexible electrical device whichcomprises

(a) an elongate flexible PTC element composed of a PTC conductivepolymer composition which exhibits PTC behavior and which comprises

(i) a polymer component, and

(ii) a particulate filler component which has been dispersed in saidpolymer component and which comprises a conductive filler;

(b) at least one electrode, preferably two electrodes which can beconnected to a source of electrical power and which when so connectedcause current to flow through said PTC element; and

(c) surrounding said PTC element and melt-fused thereto, a layer of asecond polymeric composition having substantially greater flexibility at23° C. than said PTC composition.

In another aspect the invention provides a method of making such adevice which comprises

(1) forming said PTC element by melt-shaping said conductive polymercomposition adjacent said electrode preferably adjacent a pair ofelectrodes so that the electrodes are in electrical contact with saidPTC element;

(2) forming on the surface of said PTC element a layer of a secondpolymeric composition having substantially greater flexibility at 23° C.than said PTC composition; and

(3) heating said layer to cause melt fusion of said second polymericcomposition and said PTC composition at the interface between said layerand said PTC element.

BRIEF DESCRIPTION OF THE DRAWING

The invention is illustrated in the accompanying drawing in which

FIG. 1 is a cross-section through a flexible self-limiting strip heaterof the invention, and

FIG. 2 illustrates a method for testing the flexural behavior of adevice.

DETAILED DESCRIPTION OF THE INVENTION

The PTC conductive polymer composition used in the present invention maybe of any kind, including those disclosed in the prior art, providedthat melt fusion thereof to the second polymeric composition can takeplace. Thus the PTC composition is preferably one that can bemelt-shaped, e.g. by extrusion, and is preferably substantially freefrom cross-linking when the melt-fusion takes place. Once themelt-fusion has taken place, the PTC composition can if desired becross-linked, e.g. by irradiation. The invention is of particular valuewhen the PTC composition is one which is relatively rigid (i.e. whichhas low elongation and/or performs poorly in a flex test of the kinddescribed below). Such rigidity can result from the amounts andcharacterisics of the polymer or mixture of polymers and/or theconductive filler (which will often consist of or contain one or morecarbon blacks) and/or non-conductive filler(s), selected for the desiredelectrical and other characteristics of the PTC composition. I havefound this invention to be particularly useful when the PTC compositionis as described and claimed in the International (PTC) application No.8,000,592 entitled "PTC compositions" filed contemporaneously herewithby the assignee of this application, Raychem Corporation; the disclosureof that International Application is incorporated herein by reference.In those compositions, the polymer component comprises a mixture of afirst crystalline polymer having a first melting T₁ and a secondcrystalline polymer having a second melting point T₂ which is at least(T₁ +25)° C., preferably at least (T₁ +70)° C.; the polymer componentcan also contain other polymers, e.g. elastomers. The mixture ofcrystalline polymers need not be a physical mixture of two distinctpolymers but may be a single polymer, e.g. a block copolymer, havingdistinct segments such that the polymer has two distinct melting points.The melting points referred to are the peak values of the peaks of a DSC(differential scanning calorimeter) curve. T₂ is preferably at least160° C., especially at least 200° C., when it is desired that thecomposition is stable on exposure to high temperatures. T₁ is selectedfor the desired switching temperature (T_(s)) of the composition, andmay be for example 100° to 175° C. One or both of the polymers may be afluorinated polymer, for example the lower melting polymer may bepolyvinylidene fluoride and the melting polymer anethylene/tetrafluoroethylene polymer.

The second polymeric composition which is applied as a layer around thePTC conductive polymer element and melt-fused thereto should havegreater flexibility than the PTC conductive polymer composition at roomtemperature (25° C.), preferably at all temperatures between 0° and thehigher of 100° C. and the T_(s) of the PTC composition, especiallybetween -20° C. and the higher of 100° C. and the T_(s) of the PTCcomposition. The second polymeric composition and the thickness of thelayer thereof should be such that it substantially enhances the flexlife of the device in a flex test of the kind described below, and inits broadest sense the term "having greater flexibility" should beconstrued accordingly. Normally the layer will be of uniformcomposition, but the invention includes for example a first layercomposed of a hot melt adhesive which is melt-bonded on one surface tothe PTC element and on the other surface to a layer of another polymericcomposition. In a modification of the invention a layer of a second,more flexible, polymeric composition is bonded to the PTC element bymeans of another form of adhesive, in which case the heating step isomitted or replaced by another step to activate the adhesive.

The second polymeric composition is often an electrical insulator and ispreferably substantially free of (i.e. contains 0 to 10% by weight of)fillers and other additives. The second polymeric composition should becompatible with the PTC composition to ensure melt-fusion, andpreferably the second polymeric composition comprises at least 50% byweight of units which are the same as units constituting at least 50% byweight of the polymer providing the continuous phase in the PTC element.Preferably the second polymeric composition contains at least 90% byweight of the polymer providing the continuous phase of the PTC element.

The layer of the second polymeric composition is preferablymelt-extruded around the PTC element, e.g. using a cross-head die. Thusin a preferred procedure, the PTC composition is melt-extruded aroundtwo (or more) electrodes, e.g. to provide the core for a self-limitingstrip heater, and the layer of the second polymeric composition isapplied thereto by simultaneous or subsequent melt extrusion, e.g. bycoextrusion. The layer of the second polymeric composition need not beapplied in a separate operation and the invention includes for exampleformation of the layer by appropriate adjustment of the extrusionconditions so that the PTC element is of non-uniform composition, withthe outer layer having the desired characteristics.

When, as is preferred, the layer of the second polymeric composition isapplied separately in a melt-extrusion operation, the extrusionconditions can be adjusted so that melt-fusion takes place essentiallysimultaneously with the extrusion, so that no separate heating step isrequired. In many cases, however, a separate heating step is moreconveniently carried out. Especially is this so when it is desired toanneal the PTC composition to reduce its resistivity, in which case theannealing step is preferably carried out under conditions such that thedesired melt fusion is effected at the same time. When using a PTCcomposition as described above which comprises two crystalline polymers,annealing is preferably carried out at (T₁ +5)° C. to (T₂ -10)° C. for atime sufficient to reduce the resistivity at 25° C. of the PTCcomposition from a first value, _(o), prior to said annealing, to asecond value, e_(A), which is less than 0.8×e_(o), preferably less than0.6×e_(o), with e_(A) preferably being from 10² to 10⁵ ohm.cm. Annealingin this way is described and claimed in my copending commonly assignedapplication Ser. No. 150,911 entitled "Improved method for annealing PTCcompositions" filed contemporaneously herewith, the disclosure of whichis incorporated herein by reference.

Referring now to FIG. 1 of the drawings, wire electrodes 1 and 2 areembedded in PTC element 3, which is surrounded by, and melt-fused at theinterface to, a layer of an insulating polymeric composition 3, which isitself surrounded by a further layer of another insulating composition5. Referring now to FIG. 2, strip heater 10 is secured at its upper endto a holder 11, and a weight of 1 lb. is secured to its lower end. Thestrip heater passes between and just contacts two fixed mandrels 12 and13, each of 0.5 inch diameter. To determine the behavior of the stripheater on flexing, the holder is moved in the arc of a circle first toone side and then to the other, as shown by the dotted lines, at a rateof 40 cycles per minute. The number of cycles required to initiate andto complete breaking are determined.

The invention is illustrated by the following Example.

EXAMPLE

The ingredients used in this Example are given in the Table below.

The ingredients for Composition A were dry-blended, and the blend fed toa Werner Pfleiderer ZSK co-rotating twin screw extruder heated to about260° C. and fitted with a pelletizing die. The extrudate was choppedinto pellets.

The ingredients for Composition B were dry-blended and the blend fed toa Werner-Pfleiderer ZSK extruder heated to 315°-345° C. and fitted witha pelletizing die. The extrudate was chopped into pellets.

Two parts by weight of the pellets of Composition B and one part byweight of the pellets of composition A were dry-blended together andthen dried in air for about 16 hours at about 150° C. The dried blendwas melt-extruded at 315°-340° C. through a single screw extruder fittedwith a cross-head die around two pre-heated 18 AWG strandednickel-coated copper wires whose centers are about 0.29 inch apart, toproduce an extrudate having a cross-section of dumbbell shape as shownin FIG. 1, the distance between the closest points of the electrodesbeing about 0.235 inch the thickness of the central section (t) beingabout 0.030 inch and the thickness of the end sections (d) being about0.070 inch. After the extrudate had cooled, two jackets were extrudedaround it, the inner jacket being 0.02 inch thick and composed ofpolyvinylidene fluoride having a melting point of about 156° C. (Kynar460 from Pennwalt) and the outer being 0.025 inch thick and composed ofa fluorinated ethylene/propylene copolymer having a melting point ofabout 247° C. (Teflon FEP 100 from du Pont). The jacketed strip wasannealed at 175° C. in air for 4 to 9 hours. The product had across-section as shown in FIG. 1.

In another test which can be used to determine whether a strip heaterhas satisfactory flexibility, a length of the heater is held at one endin a fixed holder and at the other end by a rotatable holder which canbe rotated through 180° . The distance between the holders is fixed,usually at a value of 1 to 4 inch, e.g. 1.25 or 3.375 inch, and theheater is under a known tension, usually of 0.2 to 1 lb. e.g. 0.46 lb.The rotatable holder is rotated clockwise and anti-clockwise through180° at a fixed rate, e.g. 15 cycles/minute. At intervals the PTCelement is inspected to see whether it has cracked.

                                      TABLE                                       __________________________________________________________________________                           Comp. A Comp. B Final Mix                                                     Wt %                                                                              Vol %                                                                             Wt %                                                                              Vol %                                                                             Wt %                                                                              Vol %                              __________________________________________________________________________    Polyvinylidene Fluoride having a melting                                                             88.0                                                                              89.2        29.3                                                                              32.0                               point of about 160° C. (Knyar 451 from                                 Pennwalt)                                                                     CaCO.sub.3 (Omya Bsh from Omya Inc.)                                                                 3.0 2.0         1.0 0.7                                Carbon Black (Vulcan XC-72 from Cabot,                                                               9.0 8.8         3.0 3.2                                particle size 300 Angstroms,                                                  surface area 254 m.sup.2 /g)                                                  Ethylene/tetrafluroethylene copolymer                                                                        64.6                                                                              75.5                                                                              43.1                                                                              48.4                               having a melting point of about 270° C.                                (Tefzel 2010)                                                                 Carbon Black (Continex HAF from Continental                                                                  15.0                                                                              16.5                                                                              10.0                                                                              10.6                               Carbon, particle size 290 Angstroms,                                          surface area 80 m.sup.2 /g)                                                   ZnO (Kadox 515 from Gulf and Western)                                                                        20.0                                                                              7.2 13.3                                                                              4.5                                Processing aid                 0.4 0.8 0.3 0.6                                __________________________________________________________________________

I claim:
 1. A flexible electrical device which comprises(a) an elongateflexible PTC element composed of a PTC conductive polymer compositionwhich exhibits PTC behavior and which comprises(i) a polymer componentwhich comprises a mixture of a first crystalline polymer having a firstmelting point T₁ and a second melting point T₂ which is at least (T₁+25)° C.; and (ii) a particulate filler component which has beendispersed in said polymer component and which comprises a conductivefiller; (b) two elongate metal electrodes which are electricallyconnected to the PTC element so that, when the electrodes are connectedto a source of electrical power, current flows through said PTC element;and (c) surrounding said PTC element and melt-fused thereto, a layer ofa second polymeric composition having substantially greater flexibilityat 23° C. than said PTC composition.
 2. A device according to claim 1wherein said electrodes are surrounded by and in physical contact withsaid PTC element.
 3. A device according to claim 1 wherein T₂ is atleast (T₁ +70)° C.
 4. A device according to claim 1 wherein the secondpolymeric composition has a melting point of (T₁ -10)° C. to (T₁ +50)°C.
 5. A device according to claim 1 wherein the second polymericcomposition comprises said first crystalline polymer.
 6. A deviceaccording to claim 5 wherein the second polymeric composition consistsessentially of said first crystalline polymer.
 7. A device according toclaim 1 wherein said PTC composition has a resistivity of 10² to 10⁵ohm.cm.
 8. A device according to claim 1 wherein T₂ is at least 160° C.9. A device according to claim 8 wherein T₂ is at least 200° C.
 10. Adevice according to claim 9 wherein T₁ is 100° to 175° C.
 11. A deviceaccording to claim 1 wherein each of the crystalline polymers in the PTCcomposition is a fluorinated polymer.
 12. A device according to claim 1wherein the first crystalline polymer is polyvinylidene fluoride and thesecond crystalline polymer is an ethylene/tetrafluoroethylene copolymer.13. A device according to claim 1 wherein said second polymericcomposition has substantially greater flexibility than said PTCcomposition at all temperatures between 0° C. and 100° C.
 14. A deviceaccording to claim 1 wherein said second polymeric composition hassubstantially greater flexibility than said PTC composition at alltemperatures between 0° C. and the switching temperature of thecomposition.